My Iridescent Shark Catfish had many of the same symptoms. While trying to diagnose the problem I referred to those symptoms as being "social" symptoms -as though he may be afraid of other fish and so on. Then I ended up thinking about the possibility that even though this species is diurnal, my particular Shark (Bruce is his name) might be nocturnal. I began getting up earlier then usual as sort of a 'test'. When I would wake up, I would try to make as little noise as possible (Bruce spooks very easily -like all Iridescents) and not turn on any lights at all. Heck, I wouldn't even move out of bed, I would only move enough to be able to view the aquarium. generally speaking, Bruce would be out and about acting like he used to when I initially introduced him to the aquarium. He was very social, not scared of any other fish and seemed to be flat out having a blast. It got to a point where I would make it a point to leave the lights out for extra long in the mornings to make sure he got excercise as all he did all day when the lights were on is sit in this one corner of the aquarium. He did not move AT ALL. He didn't even flutter his fins to keep equilibrium, he would just kind of float around. The only notable movement was his pupils looking around now and again. When night time would come around, maybe 30 minutes before I typically shut off the light, he would come out and start swimming around like he did in the mornings (he did this about 80% of the time) so that blew my mind needless to say..... I was flabbergasted until one day a friend of mine suggested that maybe the light is hurting poor Bruce, as in the light waves are actually causing him pain..... Well, one thing lead to another and I ended up finding out that when a fish is infected with a protozoan infestation they are typically sensitive to light and prefer extreme darkness.
I treated with Aquari-sol by Aquarium Products (Copper sulfate) at the rate of 12 drops per 10 gallons and immediately noticed a difference. Within 24 hours Bruce is completely back to normal and loving life!
Don't get me wrong, I'm not saying that you should go and treat with copper sulfate just because your shark seems 'lazy' or whatever but what I AM saying is that maybe they have a protozoan disease and the light is causing them pain? This is a possibility you may want to look into.
Bruce showed some physical signs of an infestation as well, he had black peppered spots on his belly, a white slimy coating developing, deep scratches from glancing off of aquarium decorations/gravel, minor fraying off his pectoral fin and classic velvet signs like browning around the lower fins and belly which grows worse and thicker as the days go by. All of these symptoms have now disappeared and today marks the third day that I have repeated the dosage of copper sulfate.
I hope that helps. I really hate to see Iridescents in distress
Joe